Nanoparticle Diffusion Coefficients at Liquid Interfaces Measured by SEM Single Particle Tracking

Nearly monodisperse nanoparticle spheres attached as a Gibbs monolayer to a nonvolatile ionic liquid surface were tracked by a recently developed scanning electron microscopy technique to obtain their two-dimensional tracer diffusion coefficient as a function of areal fraction. To allow tracking from dilute to almost jammed, ligand-coated gold tracer particles were sparsely dispersed among silica background particles of the same diameter and surface chemistry. The in-situ technique spatially resolved both tracer and background particles for a period of ~1-2 minutes, highlighting mechanisms of diffusion, which depended strongly on areal fraction. Irrespective of diameter, beyond onset of crystallization but before jamming the diffusion coefficient decreased by over four orders of magnitude. The normalized diffusion coefficient across this range was unexpectedly large and diameter dependent, manifesting a significant lubrication of dense particle motions by the surface-attached ligands, 5,000 g/mol poly(ethylene glycol). More lubrication was found for a longer ligand.